qBittorrent 5.2.0 changed /api/v2/auth/login to return HTTP 204 (No Content)
on success instead of HTTP 200 with body 'Ok.'. The adapter required the body
to equal 'Ok.', so every login on 5.2.0+ failed with 'HTTP 204 body=' — the
connection probe and all torrent actions were broken.
Treat login as successful on the SID auth cookie and/or a success body: 'Ok.'
(<=5.1) or an empty HTTP 204 (>=5.2.0). Still reject bad creds, which
qBittorrent reports as HTTP 200 + 'Fails.' (not a 4xx).
Tests: 204-empty -> success, SID-cookie+empty-body -> success, 'Fails.' (even
with a stale cookie) -> failure.
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